1. Field of the Invention
The disclosed method and apparatus relate to systems for studying air quality. More specifically, the invention relates to low-volume, high-volume and passive atmospheric sampling for the determination of semivolatile organic chemical (SOC) concentration.
2. Background Art
Semivolatile organic compounds (SOCs) are typified by low volatility, exhibiting vapor pressures from 101 to 10−6 Pa and consequently low vapor-phase concentrations in ambient air (concentrations of ca. pg m−3; mixing ratios of ca parts-per-quadrillion by volume (ppqv)). Synthetic SOCs are used in a wide range of products such as pesticides, flame retardants, plasticizers, and lubricants. Many SOC's are also classified as persistent bioaccumulative toxicants (PBTs) because of their long environmental lifetimes (persistence), their tendency to accumulate in the tissue of animals (bioaccumulate), and adverse health effects to humans or other organisms (toxic). Anthropogenic and synthetic SOCs also influence climate through reactions with organic particles.
As awareness of the toxic effects and relevance to climate of SOCs has grown, so has the need to understand their transport and fate in ambient air. In addition, because vapor-phase and particle-bound SOCs may have significantly different transport mechanisms, health impacts, and influences on climate, accurate separation and collection of vapor-phase and particle-phase SOCs is desirable.
SOC sampling techniques include active and passive methods. The two most common methods are ‘high volume’, an active method, which samples large quantities of ambient air, e.g., from 400 to 4000 m3, in order to collect sufficient quantities of analytes, and various passive methods that rely on diffusion of SOCs into the trapping medium to gather analytes in the absence of advection. These techniques typically employ solvent extraction to liberate the collected analytes from the sampling media, which results in sample dilution and an increase in both the required sample volume and time required to gather that volume. Separation of vapor and particulate phases is not possible with passive samplers, which collect primarily gas-phase compounds. High-volume samplers, when employed in their typical configuration with a particulate filter anterior to the vapor phase sorbent, suffer from sampling artifacts that may confound the differentiation between gas- and particle-phase SOCs.
Conventional High-Volume Sample Collection and Analysis by Soxhlet Extraction, Column Chromatography, and Concentration. This sampling and analysis technique has been in use for ca. 30 years to measure ambient air concentrations of semivolatile organic compounds, and is considered to be the “workhorse” of SOC monitoring programs world-wide. It involves collection of particle-phase compounds on filters followed by collection of gas-phase compounds in XAD resin or polyurethane foam (PUF) plugs at a flow rate of from 40-1000 L min−1 (typically 700 L min−1). The resin or PUF plugs and filters are subsequently processed using Soxhlet extraction, column chromatography to clean up the extract, and concentration of the solvent extract by roto-evaporation and/or solvent blow-down. The mass of analyte in the extract is then analyzed by injecting a small volume of the extract (ca. 1/100th of the solvent volume) into a gas chromatograph.
There are certain problems with prior approaches:
1. It is known that gaseous and particle-phase chemicals are incompletely separated using high-volume air sampling. Research directed at quantifying and correcting for the artifacts has been conducted. The particle-phase compounds can be desorbed during the long sampling times required (hours to days), in part due to diurnal temperature changes during sampling, while some gas-phase compounds can sorb to the filter.
2. There is a significantly increased opportunity for losses to occur during the analytical procedures to extract and analyze the compounds as compared to, for example, thermal extraction into minitubes and subsequently into the gas chromatograph.
3. Because only a small portion of the analytes collected are actually injected into the gas chromatograph, the sampling time must be correspondingly longer to collect adequate sample mass at the comparable flow rates that are employed (40-1000 L min−1). Analyte concentrations and meteorological conditions can change over the sample collection time, with consequent losses in resolution of analyte concentrations in a sample as well as introduction of potential artifacts due to changing sorbent properties within the sampler under varying meteorological conditions.
4. Many of the prior art sample processing steps are relatively expensive and they entail an increased sample turn-around time.
5. High-volume sampling and analysis require significant amounts of solvent and XAD resin, neither of which can be re-used (unless PUF is used as a sorbent rather than XAD), and thus these techniques are less sustainable.
Passive Sampling and Analysis of Combined (Gas and Particulate) Semivolatile Organic Chemicals by Soxhlet Extraction, Column Chromatography, and Concentration. Passive sampling has been utilized to collect SOCs in ambient air. This technique is similar to high-volume air sampling except that compounds are collected in PUF by passive diffusion in an estimated volume of air sampled over a time period of weeks to months, rather than by actively drawing air through XAD resin or PUF. The required sample processing steps are the same as for analysis of XAD resin or PUF as described above. A recently developed version of a passive sampler, a “flow-through” sampler allows considerable shortening of the required sampling time, but the other issues with passive sampling still exist using this version.
Although passive sampling produces a concentration averaged over the entire sampling period, that period is on the order of weeks to months. No information can be gathered on short term variations in concentration of collected compound concentrations. In addition, variations in meteorological conditions during lengthy passive sampling periods can lead to variations in amount of SOC collected, making spatial comparisons of estimated concentrations inappropriate over large regions where local meteorology differs. Thus, application of passive sampling technology must be limited to cases in which its constraints are acceptable.
Certain disadvantages of passive sampling and analysis include increased opportunity for losses to occur during the analytical procedures to extract and analyze the compounds as compared, for example, to thermal extraction into minitubes and subsequently into the gas chromatograph, higher expense of analysis, greater time required for analysis, consequent increased sample turn-around time, and decreased sustainability of solvent usage.
Diffusion Denuders. A “denuder” is a device that traps gas and allows particles to be collected separately. Diffusion denuders have been developed in various shapes and sizes for analysis of charged and neutral chemicals including annular, parallel plate, capillary, and honeycomb structures. Some diffusion denuders can be coated with various sorbents including silicone grease, crushed Tenax-GC™ and Florisil, silicone gum, and crushed Tenax-GC™ applied onto silicone gum. In addition, various analyte extraction techniques have been attempted including solvent extraction, supercritical fluid extraction, and thermal desorption, but solvent extraction is typically employed.
It is known to use sticky resin beads whose pores are sized so as to trap molecules of organic gases—small enough to adhere through friction alone to a sand-blasted inner surface of a glass tube. Science Beat, Berkeley Lab Air Sampler Zeros In On Atmospheric Pollutants”, Sep. 1, 1999 (reporting the work of Lara Gundel). In operation, solid particles are relatively massive and travel straight through a denuder. Gas molecules, however, eventually hit a wall and stick. Depending on air flow and the length of tube, particles may stay airborne, but long enough for gas to become trapped. Id.
In one approach, after an air sample is sucked through the denuder, the particle filter is removed and gas trapped on the resin beads is extracted and analyzed. Capillary diffusion denuders are known that use sections of commercial gas chromatography, fused silica capillary columns bundled together and encapsulated with expoxy. However, cracking of the epoxy during repeated thermal cycling has recurred.
Annular Denuder System. The prior art includes diffusion denuders made from annular sandblasted glass channels coated on the inside with ground XAD-4 resin. See, e.g., U.S. Pat. Nos. 6,226,852 and 6,780,818. The systems are presently marketed containing uncoated annular denuders by URG Corporation (Chapel Hill, N.C.: http://www.urgcorp.com/), but denuders coated to specification alone were previously marketed by Restek, Inc. These annular diffusion denuders must be recoated after being used 20 times for sample collection and solvent extraction of analytes.
Several technical challenges are therefore presented by prior approaches:
1. Design of a durable sampling device utilizing diffusion denuders for separation of gaseous SOCs and particle-associated SOCs that can be manufactured inexpensively from commercially available materials. Ideally, the SOC sampling device should have the capability to enable analytes to be extracted thermally into an analytical device to measure the gaseous SOCs and particle-associated SOCs at concentrations found in ambient air.
2. Design of a system to transfer and concentrate analytes by thermal extraction (analyte transfer apparatus). It would be desirable to have a system that accommodates the need for a high flow rate to extract analytes from the larger diameter denuder (several hundreds of liters per minute), while achieving quantitative and complete transfer of analytes into the analytical column in the gas chromatograph, which can only accept flows of ca 2 mL min−1.
3. Design of a hot gas spike apparatus to uniformly distribute surrogates and standards in denuders in the gas phase.
4. Development of custom-packed minitubes and gas chromatograph inlet liners. Ideally, such minitubes and liners should be capable of trapping more-volatile SOCs (e.g., hexachlorocyclohexane) while having low reactivity and the capability to quantitatively release reactive, low-volatility SOCs at high temperatures (300° C.).
Thus, conventional high-volume and passive atmospheric sampling (indoors and outdoors) for determination of semivolatile organic chemical (SOC) concentration in gas and particle-associated phases require long sampling periods because only a small portion of the analytes collected are typically analyzed in a gas chromatograph, or in the case of passive and flow-through sampling, no phase separation.